1. Field of the Invention
Cables of this type are generally known under the designations "glass fiber cable" and "light transmission cables", both of which are not entirely correct inasmuch as the fiber material of optical fibers, although being made of glass as a rule, can however also be made of other, special plastic materials for optical fibers, and also because signal transmission through optical fibers is no longer limited to the range of visible light but extends on the one hand into the area of infrared heat radiation and on the other hand into the area of invisible ultraviolet radiation. The designation "optical fiber" which is now in general use and is also used here, is accordingly to be understood in the sense of a fiber for signal transmission by means of electromagnetic radiations transmitted in the longitudinal sense of the fibers, without any limitation to visible light as the light transmitter, as might be deducted from the term "optical".
2. Description of the Related Art
Cables of the above-mentioned type, with optical fibers for the transmission of signals and the essential requirements to be met in the design of such cables are known, as for example from the literature such as "Introduction to optical fiber cables" (Pirelli Cable Review No. 26, Jul. 85, pages 1-6) and "Design considerations for optical cables" (Pirelli Cable Review No. 26, Jul. 85, pages 52-56). One of the most important requirements for the design of such a cable is that the optical fibers must be protected from any kind of axial tension load as well as from extreme bending. However, these most important requirements were precisely the ones that could only be met to an insufficient degree until now or were simply not met in the known light transmission cables under certain unfavorable circumstances or when several such unfavorable circumstances, varying from cable to cable, came together, even though it does not appear to be especially difficult at first glance to meet these requirements of complete protection of the optical fibers from axial tension loads and extreme bending because axial tension loads for instance are or seem to be preventable by a helical course of the optical fibers or a stranding of the cable and extreme bending by an appropriate armoring of the cable.
Closer investigation shows however that the originally planned helical course of the glass fiber cable does not provide sufficient protection of the fibers from axial tension load because with such a course, although in the presence of curvatures in the cable the shifting of fiber segments which are on the inside of the curvature to the outside of the curvature results in a compensation of length and although the axial tension load of the fibers, when the cable is bent, is limited to the tension loads which are required for this shift and which are generally relatively small, axial tension loads affecting the fibers occur as a rule when thermal expansion of the cable takes place, such as for instance as a result of sun irradiation upon an outdoor cable, capable of bringing about a rupture of the fibers because the thermal expansion coefficient of glass is known to be extremely low and because the thermal expansion coefficient of the entire cable is therefore as a rule considerably higher than the thermal expansion coefficient of the fibers; this results, when thermal expansion of the cable occurs, in axial elongation of the helical line of the course of the fibers and would therefore require a decrease of the diameter of the cylinder on which the helical line runs in function of this axial elongation, less the minimal fiber elongation resulting from the thermal expansion of the fibers, whereas such a decrease of the diameter is not possible because the guidance means provided inside the cable to guide the fibers in a helical course support the fibers radially and thereby render impossible a shifting of the fibers towards the center of the cable, and furthermore because of the thermal expansion of the cable which naturally takes place in the transversal direction of the cable and not only in its longitudinal sense, actually causing a shifting of the fibers away from the center of the cable and therefore an increase of the diameter of the cylinder on which the helical line takes its course, thereby forcing an elongation of the fibers which exceeds their upper breaking elongation limits and thus causing their rupture.
Neither can the danger of fiber breakage due to thermal cable expansion be eliminated by providing a helical course of the fibers with periodically alternating sense of twist along the cable instead of a helical course of the fibers with a sense of twist remaining constant over the entire length of the cable, according to the so-called false-twist method. For even when the fibers take such a course, axial elongation of the helical line with periodically alternating sense of twist results naturally from thermal cable expansion, provoking a tendency for a decrease in diameter of the cylinder on which the helical line with periodic alternating sense of twist takes its course, as a shift of the ranges of the alternation of the sense of twist of the helical line in the circumferential sense of the cable is impossible because of the guidance means which are naturally also required when the course of the fibers is helical with periodically alternating sense of twist, and because the guidance means support the fibers radially in the same way as with a helical course with constant sense of twist the above-mentioned shrinking tendency of the diameter of the cylinder on which the helical line with periodically alternating sense of twist runs leads to a rupture of the fibers, just as is the case with a helical course of the fibers with constant sense of twist.
The theoretically possible utilization of materials with different expansion coefficients for the optical fibers in order to prevent the danger of fiber rupture as a result of thermal expansion of the cable is not possible in practice for a number of different reasons. The main reason is first of all that no material with a higher expansion coefficient and suitable for optical fibers is known. But even if such materials were known, the danger of fiber rupture could be avoided through the utilization of a fiber material with different expansion coefficients only if the expansion coefficients of the fiber material and of the entire cable at least nearly coincided, and to obtain such a coincidence is not possible in practice because the expansion coefficient of the entire cable can be vary greatly, depending on the composition of the cable and on each of the materials used in the cable, and because with a relatively high expansion coefficients of a fiber material and the expansion coefficients of a cable being relatively low, a considerable difference in expansion coefficients would again result which, in this case, would lead to a rupture of the fibers not when the cable is heated excessively, but rather when it is cooling down to a great degree.
Thus, with the fibers following a helical course, the danger of fiber rupture cannot be reliably eliminated either with a constant nor with a periodically alternating sense of twist of the helical line because the requirement for the protection of the optical fibers from axial tension loads cannot be met with this stranding principle of design, in any case not if the cable is exposed to relatively high temperature stresses, such as for example the operating temperature range of -40.degree. C. to +70.degree. C. prescribed for outdoor cables.
There exist therefore relatively narrow limits for admissible temperature ranges for stranded cables with optical fibers already in operation or on the market, limits that can be observed in practice only with cables running underground, and furthermore the optical fibers in stranded cables, in addition to severe temperature conditions, would have to be running loosely in relatively hard tubes so that the earlier-mentioned shifting of the fiber segments which are on the inside of the curvature towards the outside of the curvature may take place for the sake of length compensation when the cable is bent, without imposing great tension loads upon the fibers as would be the case in the absence of these relatively hard tubes if the fibers were to be held back by internal pressure of the cable and if said shifting could therefore only occur in opposition to considerable friction resistance. Since the hard tubes must resist all possible internal cable pressures during operation and when the cable is laid, and furthermore must be provided with a sliding layer made of a high-resistance smooth synthetic material, e.g. nylon to avoid a so-called micro-bending of the fibers against rough surfaces on the inside of the tube wall, the manufacture of these relatively hard small pipes is extremely costly, so that stranded cables with optical fibers, in addition to the disadvantage of low temperature tolerances, have the disadvantage of being very expensive to make.
It is obvious that these decisive disadvantages of the stranding design principle in cables with optical fibers had to lead to attempts to meet the requirement of complete protection of the optical fibers from axial tension loads and extreme bending by applying other design principles to the building-up of cables with optical fibers. Thus, for example, the British patent application GB-A 2,122,767 proposed to imbed the optical fibers along a wavy course in rubber and to combine a number of such rubbers with fibers running along a wave-shaped course into one package and to twist the latter too, if necessary. This proposal was not sufficiently thought through however, because a wave-shaped fiber course would only make sense if a fiber could go over into a straight-line or nearly straight-line state without difficulty, i.e. without axial tension loads, and in order to achieve such a straight-line course the fibers imbedded in rubber along a wavy course would have to cut through the rubber in the plane of the wave-shaped course, and such cutting would of course (if at all) be possible only with extraordinarily high tension loads being applied, whereby the fibers would as a rule break before these are reached. This can be seen easily if one attempts to cut through a soft eraser with a tensioned steel wire (without moving the rubber back and forth in the axial direction of the wire), for in such a test the rubber is not cut but the steel wire tears. Aside from this, with the layout according to the above-mentioned British application, the space required per fiber, being approximately 1000 times the volume of the fiber, is definitely excessive. A similar proposal contained in the French patent application FR-A 2,509,480 also provides for a wave-shaped course of the fibers, whereby each individual fiber is located in a kind of flat pipe with two flat pipe wall sections facing each other and with two half-round pipe wall section also facing each other and connecting these flat pipe wall sections to each other. Although in this arrangement the fiber is able to go over from its wave-shaped course to a straight-line course, in contrast with the arrangement of the above-mentioned British application, without any axial tension load for as long as the flat pipe or pipes are still straight, this state exists practically only during the manufacture of the cable, for upon completion of the manufacturing process it is already wound up on a cable roll, and it must then be possible for the flat pipes to be bent together with the cable so that for that reason they may not be so hard as to break when bent in this manner. The flat pipes must therefore be made of a flexible material, and if this is the case, they will be flattened within their elastic range of deformation in the areas of curvature of the cable because flat pipes, as is known, are less stable than cylindrical pipes when subjected to outside pressures. This flattening within the elastic range of deformation does disappear again as soon as the cable is laid out in a straight line, but occurs again at points where the cable must be laid in a curve or bent, and as a result of this flattening the fibers following a wave-shaped course within the flat pipes are retained or held by pressure in the curvature zone of the cable between the flat pipe wall parts, so that the length compensation which is required precisely in the areas of cable curvature between the fibers at the inside of the curve and the fibers at the outside of the curves cannot take place, or so that said passage of the fibers from a wave-shaped course into a straight-line course is hindered or even rendered impossible and in any case entails considerable axial tension loads acting upon the fibers, capable of bringing about a breaking of the fibers taking this wave-shaped course in case of marked flattening of the flat pipes or if the fibers following a wave-shaped course inside these pipes are completely held fast through pressure, i.e. in the areas of marked cable curvature. Neither the proposal according to the above-mentioned British application, nor the proposal according to the preceding French application is therefore able to meet the requirements ensuring protection of the optical fibers under all circumstances from tension loads and thereby from the danger of breaking and this also applies to all other proposals which have become known in this respect. Thus, for example, a proposal according to the French patent application FR-A 2,534,385 provides an essentially cylindrical holding device made of a thermoplastic material for each individual fiber and surrounding said fiber, said holding device being provided with a slit extending across approximately 85% of its diameter and thus dividing the holding device into nearly two half cylinders to receive the fibers and being provided with two reinforcing wires, but in this proposal too, just as with the flat pipes discussed earlier, the danger exists that the holding device may be flattened or that the slit may be pressed together, thus causing the fibers to be pinched in the slit, which could then result in a tearing of the fibers in the areas of curvature of the holding device. It is true that this danger of the slit being pressed together with one single such holding device does not exist because the two reinforcement wires only permit bending of the holding device that exclude compression of the slit, but a cable with optical fibers not only contains one single, but several optical fibers as a rule and would accordingly also have to contain a plurality of such holding devices for the individual fibers, and then the internal pressures within the cable which cause such a compression of the slits, especially in the areas of curvature of the cable, could no longer be avoided. Also different other published proposals for the meeting of the above-mentioned requirements, such as for example the proposals according to the Australian patent application AU-A 543574 and according to the French patent application FR-A 2,489,002, provide for a helical course of the optical fibers in the longitudinal sense of the cable by means of which, for reasons already discussed in greater detail earlier, said conditions can in any case not be met. These proposals for fibers following a helical course do as a rule provide special measures to avoid the danger of tearing the fibers; the above-mentioned Australian application for example provides for the manufacturing of the cable at a temperature close to the upper limit of the operating temperature range, so that the entire cable contracts more than the fibers at median operating temperatures, resulting in the fibers becoming longer than the cable at median and low operating temperatures that may occur, and the French application 2,489,002 does for example provide for an elastic support of the fibers within the cable radially, by means of an intermediate layer of foamed material between fibers and fixed radial support means, but these measures do not suffice to completely remove the danger of fiber rupture and are especially unsuitable to protect the optical fibers according to the above-mentioned requirements from all axial tension loads, i.e. also from axial tension loads short of provoking fiber rupture.
Seen as a whole, it can be said about the state of the art as it pertains to cables of the type mentioned initially that it has not been possible until now to meet the requirement, under all conditions and combinations of conditions, that the optical fibers be protected from all axial tension loads and also from stronger curvatures such as the so-called micro-curvature could not yet be met completely until now with cables of the type mentioned initially.